1. Field of the Invention
This invention relates generally to apparatus for logging downhole conditions in a well and more particularly, but not by way of limitation, to sonic logging tools which emit sonic pressure waves and receive reflections or refractions thereof from downhole features in the well for determining the nature of such features.
2. Description of the Prior Art
In the oil and gas industry one manner in which reservoir management and well maintenance are performed is to conduct sonic logging tests wherein sonic waves are emitted by a transmitter located within a tool positioned in the well and then received by a receiver located within the tool after the waves have been reflected or refracted by the downhole features of the well. The received sonic waves are converted into an understandable log so that information about the downhole structure can be learned. For example, such a log can be used to disclose the nature of the bond which exists between the casing and the bored surface of the well. To perform this sonic logging, a sonic logging tool having a transmitter for generating and transmitting the sonic waves and a receiver for receiving the reflected or refracted waves is needed.
Because of the extreme conditions (such as high pressures and temperatures) which may be encountered at downhole locations, the sonic logging tool needs to be rugged to withstand these conditions. Additionally, because the diameter of a well or the tubing positioned therein can vary, the sonic logging tool needs to be compact so that it can be used in both small and large diameter structures. Additionally, a ruggedly and compactly constructed logging tool is desirable because it can be more economically constructed and maintained.
Because both the transmitter and receiver are contained within the tool, it is also necessary that the tool include means for isolating these two components so that the sonic waves emitted by the transmitter do not travel in substantially detectable quantities through the tool structure to the receiver. It is desirable to provide an effective sonic isolation means over a relatively short length of the tool because the shorter the isolator, the more manageable, and likely more inexpensive, is the tool. Although it is desirable for the isolation portion of the tool to be relatively short, this portion must maintain effective mechanical coupling between the transmitter and receiver portions of the tool so that the lower of these portions will not separate from the upper portion and drop into the well thereby necessitating costly fishing trips into the hole to retrieve the separated portion.
To generate and transmit the sonic waves, it is further desirable for the tool to include an electromagnetically responsive element which changes diameter in response to an appropriate current flowing through an electrical winding or coil wrapped around the electromagnetically responsive element. Because such a wave generating means has the shortcoming of "ringing", or producing secondary waves which interfere with the primary wave created by the initial current, it is necessary for the tool to include means for providing a damping shorted turn effect whereby the "ringing" of the electromagnetically responsive element and winding are attenuated. However, this ringing attenuating means should not weaken or add bulk to the tool because it is desirable for the tool to be made rugged and compact as previously described.
One type of sonic logging tool known in the art includes a transmitter having a band disposed around a portion of the tool structure and within an energizing coil. Wrapped around the energizing coil is a shorting coil which provides a path for conducting a protective current by which the damping shorted turn effect is created. This shorting coil causes this type of tool to be relatively bulky and to also be susceptible to failure because of the intrinsic weakness of a coil of wire. Therefore, such a tool does not meet the needs of compactness and ruggedness.
Sonic logging tools known in the art also include isolator joints; however, these joints are made of lead or flexible strips, for example, and physically separate the transmitter and receiver by a relatively long distance, such as three feet. Therefore, one of these relatively long tools is likely costlier than a shorter tool because of the additional material and are less manageable than a shorter tool because of the extra length and weight or flexibility of the isolator joint. Therefore, these types of tools do not provide relatively short sonic isolator joints which maintain an effective mechanical coupling between the transmitter and receiver portions of the sonic logging tool.